Method for Frequency Planning of a Cellular Radio System with IRC

ABSTRACT

In a method and a device for frequency planning of a cellular radio system with IRC. a cost function is provided that takes the IRC capabilities of the cellular system into account. Using the method and the device the frequency optimization can be made to take into account systems employing multi carrier techniques whereby the planning complexity and time for operators is reduced.

TECHNICAL FIELD

The present invention relates to a method and a device for assigningfrequencies of a frequency plan. In particular the present inventionrelates to a method and a system for assigning frequencies in a cellularradio system employing some kind of Interference Rejection Combining(IRC) or Interference Cancellation.

BACKGROUND

Enhanced Data rates for GSM Evolution (EDGE) Evolution is currentlybeing standardized in 3GPP Rel-7. The work items include higher ordermodulation, dual carrier transmission downlink, reduced latency anddual-antenna terminals which in 3GPP is referred to as MSRD—MobileStation Receive Diversity. A dual-antenna terminal is capable ofInterference Rejection Combining (IRC) in the same way as a BTS withreceiver diversity. IRC can efficiently suppress interference from asingle, dominant co-channel interferer with up to 20 dB. In scenarioswith multiple interferers, however, the benefit of IRC is lower.

There are also algorithms for Single Antenna Interference Cancellation(SAIC) that can suppress interference, but with a lower gain. Similar toIRC, one dominant interferer rather than multiple interferers isbeneficial also for SAIC. Hereafter, only IRC is used but it should beunderstood that the basic characteristics are similar, regardless if theinterference rejection is done with one or multiple antennas.

Automatic Frequency Planning (AFP) is used by major operators tosimplify the frequency planning and achieve low interference in thenetwork. These operators typically have lots of spectrum. The AFP isusually done by smaller companies providing state of the artoptimization algorithms.

The optimization in AFP is typically done by assigning costs todifferent aspects/parameters of the frequency planning, e.g. highco-channel interference can typically be given a very high cost.Thereupon, the total costs for all different aspects/parameters assigneda cost is minimized by a optimization algorithm that varies thedifferent aspects/parameters and finds the global (or local) costminimum for the area that is frequency planned.

However, existing AFP algorithms only minimize the global interferenceand will not consider potential benefits of IRC (or SAIC). The AFPoptimization might suggest a frequency plan with many moderateinterferers instead of a single strong interferer. This will not yieldthe best performance with EDGE Evolution and IRC.

A solution to utilize IRC without the need for any frequency planningconsideration would be to re-use a frequency within a cell and then noAFP changes would be needed. But there are several drawbacks with thissolution. For example, the maximum perceived C/I, i.e. afterinterference rejection, that can be obtained from two equally strongcarriers is equal to the maximum level of interference suppression, e.g.at best 20 dB. Even this best case is not sufficient for EDGE todaysince EDGE can benefit from C/I levels up to around 30 dB and it wouldbe even further below the maximum performance possible with EDGEEvolution.

Also, for legacy reasons, at least three channel groups are needed inthe cell, i.e. two for IRC capable mobiles and one for mobiles withoutIRC. It is not possible to have the same frequency appearing twice in achannel group. Therefore, two groups are needed for IRC mobiles. Withoutreusing frequencies from own cell, only two channel groups are neededi.e. one for IRC and one without IRC. If the two channel groups used forIRC do not have exactly identical frequencies then this will impact thefrequency hopping diversity since the hopping length of each group willbe shorter with three compared with two groups per sector. In addition,at least two Training Sequence Codes (TSC) per cell and at least twoHopping Sequence Numbers (HSN) per cell are required, instead of one ofeach yielding a more complex Hoping Sequence Number/Mobile AllocationIndex Offset (HSN/MAIO) planning. Finally, an extra antenna per sectoris needed to transmit IRC channel groups on separate antennas.

Considering the drawbacks outlined above, it is not recommended tore-use the same frequency in a cell compared to allowing a stronginterferer from another cell. Hence, there is a problem of how togenerate frequency plans that take into account the potential benefitsresulting to the introduction of IRC (or SAIC).

SUMMARY

It is an object of the present invention to overcome or at least reducesome of the problems associated with the introduction of IRC (or SAIC)in a cellular radio system.

It is another object of the present invention to provide a method and adevice that is capable to generate optimized frequency plans forcellular radio systems employing IRC (or SAIC) techniques.

These objects and others are obtained by the method and device as setout in the appended claims. Thus, by introducing a cost function in anautomatic frequency assignment technique, which takes into account theproperties of IRC (or SAIC) better frequency planning can be obtained.For example when running an AFP optimization algorithm, the cost for asingle strong interferer may be reduced compared to having many moderateinterferers.

Using a frequency planning device that includes a cost functionreflecting the existence of IRC (or SAIC) in a cellular radio systemwill allow for frequency planning tools to generate improved frequencyplans compared to what is currently possible.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be described in more detail by way ofnon-limiting examples and with reference to the accompanying drawings,in which:

FIG. 1 is a general view illustrating a tool for assigning frequencies.

FIG. 2 is a flow chart illustrating a procedure for frequency planning.

DETAILED DESCRIPTION

In FIG. 1 a general view of a tool 100 used for aiding in frequencyplanning is shown. The tool comprises an input terminal 101 forreceiving data related to the system that an operator is to frequencyplan. The tool, 100 also comprises a user input terminal 103 via whichterminal 103 a user can input user specific data such as assigningdifferent costs for different interferences, see below. The terminal 103can also be used for stopping the execution of different optimizationprocedures executed by the frequency planning tool 100 at differentstages as is described more in detail below. The input terminals 101 and103 are connected to an optimization module 105. The optimization module105 comprises a computer designed to execute different optimizationprocedures programmed into the computer in accordance with the inputdata received from the input terminals 101 and 103. The optimizationmodule 105 is further connected to an output terminal 107. The outputterminal 107 can for example be a screen that can be viewed by a user ofthe tool 100. The output terminal can also be a general data outputterminal, or it can be both a screen and a data output terminal.

In order to fully benefit from Interference Rejection Combining (IRC)and Single Antenna Interference Cancellation (SAIC) a cost function isincluded in the frequency assignment process to enable improved andoptimized IRC performance (e.g. for EDGE Evolution). This is obtained bya method where frequency assignment and AFP is based upon IRCcapabilities (e.g. from 3GPP).

If the IRC capabilities are not properly taken into account, thefrequency assignment and AFP would result in less optimal performancewith IRC. Thus in accordance with one embodiment of the presentinvention an input based upon IRC capabilities (e.g. from thestandardized performance in 3GPP) is used in the frequency assignmentand AFP optimization.

For example, the AFP algorithm can be adapted to reduce or remove thecost for a single co-channel interferer as compared to having manymoderate interferers. In one exemplary embodiment this is done byremoving or reducing the cost for interferers to a lower level. e.g.where it corresponds to 20 dB lower interference for single co-channelinterferers.

In the frequency assignment procedure the cost for assigning frequenciesto a first cell A can e.g. be described on a radio level. This meansthat the total cost for the network is the sum over all radios in thenetwork. The radio cost can be described as:

C _(1A) =g _(1A)(f)  (eq. 1)

Where:

C_(1A)=the cost of assigning a frequency to a radio 1 of a cell Ag_(1A)(f)=the cost function for assigning frequencies to radio 1 of acell Af=Frequency to be assigned to the radio 1 of cell A

In accordance with one exemplary embodiment C_(1A)(f) may be calculatedas:

$\begin{matrix}{{C_{1A}(f)} = {{P_{1}*{\sum\limits_{\substack{k = {all} \\ {interfering}\mspace{14mu} {cells}}}\left( c_{A,f,k} \right)}} - {P_{2}*{\max \left( \sum\limits_{\underset{{interfering}\mspace{14mu} {cells}}{k = {all}}} \right)}*\left( {1/G_{IRC}} \right)}}} & \left( {{eq}.\mspace{14mu} 2} \right)\end{matrix}$

Where:

P_(i)=User configurable priority settings for adjusting the total costlevel. For example, the settings may be used to increase cost for aradio, sector or number of sectors relative other radios, sectors ornumber of sectors.c_(A,f,k)=Interference cost generated on radio 1 when using frequencyfin cell A from interfering cell k. Note that c_(A,f,k) may be the sumof several types of cost functions such as interference cost andneighbor cost. For example, reusing a frequency may cause multiple costsfrom a reusing sector, i.e., there could be neighbor and interferencecosts at the same time. In case of IRC, all costs associated with asector can be set to be affected.G_(IRC)=Gain from IRC in linear scale, e.g. according to standardizedperformance in 3GPP. For example, 3 dB means that half of theinterference will be removed by IRC. Moreover, G_(IRC) could typicallyvary and it depends on the interference level and the number ofinterferers and can e.g. be 20 dB for a single dominant co-channelinterferer and typically less for scenarios with multiple interferers.This means that G_(IRC) can be calculated considering the distributionand values of the (Σc_(A,f,k)) term.

The second term in the equation 2 above hence reduces the cost inrelation to the IRC performance and the level of interference.

In accordance with one embodiment of the present invention, the AFPalgorithm can be adapted to consider the benefit of IRC when finding theminimum total cost and thereby achieving a solution which optimizesperformance for systems with IRC.

In the second term above as set out in equation 2, the maximuminterferer cost for a relation between a target and an interferingsector is reduced by the IRC gain factor. In accordance with oneembodiment more than one cost can be reduced depending on the IRCcapabilities of the system. For example, in a cellular radio systemcomprising IRC with three antennas, a maximum of two dominantinterferers can be reduced. In such a configuration the formula abovecan be modified to take this into account, for example by reducing thecost for those dominant interferers to a low value or even zero.

Furthermore, in existing AFP methods, only the downlink isoptimized/considered. In some situations the IRC consideration abovewill have implications also on the uplink performance. The reason isthat if a cell is interfered in the downlink it can, at the same time,cause strong uplink interference to the downlink interfering cell. Toprotect the uplink, the IRC considerations above the maximum term can beset to be applied only if the interfering cell has IRC capabilities atits base station. In that case, the uplink IRC at the downlinkinterfering cell can remove the uplink interference caused by thedownlink interfered cell.

Another aspect of the present invention is that AFP can be used forTraining Sequence Code (TSC) planning. TSC planning aims at achievingorthogonal interferers. This is also needed by IRC to identify and beable to suppress interferes. Therefore, the IRC reduction term ispreferably not included in TSC optimization since it is desired toidentify strong interferers that need a different TSC for proper IRCoperation. Hence, the second term (P₂* . . . ) in eq. 2 above isconsidered when TSC optimizing. Moreover, in accordance with oneembodiment of the present invention, the TSC optimization is performedafter frequency optimization.

The optimized frequency plan usually results in a non-zero interferencecost, i.e. it includes frequency reuse (violations) between somesectors. In the TSC optimization only costs between reusing sectors canbe considered. The TSC optimization can be used to further reduce thecost by assigning TSC so that reusing sectors use different TSC.

In an exemplary embodiment, two sectors with the same frequencies butwith different TSC are given a TSC cost of 0 (i.e. a result of no TSCreuse). Sectors with different frequencies are not given any cost at allsince only co or adjacent frequency reuse is of interest for TSC. Duringthe frequency optimization, some costs are not considered due to IRCconsiderations as above. It is implicitly assumed that different TSC areused for IRC to work accordingly. As a result, the TSC optimizationpreferably is adapted to consider all available frequency reusecosts/violations and not include an IRC reduction term as above such asthe second term of equation 2 above, i.e. the term (P₂* . . . ) in eq. 2above.

Typically, input regarding IRC capabilities is needed in the AFP toperform the above steps. The information can for example be whichtransceivers that will carry IRC capable mobile stations, the IRCchannel groups, and which sectors that employ IRC in the uplink at thebase stations.

In FIG. 2 a flow chart illustrating a procedure for frequency planningis shown. In a first step 201 data is input and analyzed to ensure thatdata input is correct. Also a model is constructed. The input data mayfor example be data related to the site, transceiver data, interferencedata, hand over data and other data that may be relevant to take intoaccount when frequency planning. In particular data can include IRCcapabilities. The model is constructed using the available specifiedspectrum. Transceivers having similar properties may also be groupedtogether. For example all BCCH radios may be grouped in one group inorder to facilitate allocation of interference costs.

The modeling in step 201 also includes specifying the interference costsand deciding which cost that is to be given the highest cost. Specifyingcosts is typically an important step which may have to be revisited atlater stages during frequency allocation. The allocation of costs mayfor example have to be revisited if it turns out that an optimizedfrequency plan has undesired effects. One such example might be that ifthere is a reuse of frequencies for neighboring cells and such a plan isundesired, the cost for handover violation can be increased. Inparticular the allocation of cost can includes a cost which takes theIRC capabilities into account. For example any of the methods describedhereinabove may be used. In addition user specified parameters such asP₁ and. P₂ in eq. 2 above and GRIC may be specified by a user in step201.

Next, in a step 203, a frequency optimization algorithm is executedbased on the modeling parameters specified in step 201. Thus, the costincluding the cost defined by eq. 2 above is calculated using anoptimization tool. In a typical optimization tool the cost is displayedto a user on a display such that the user can stop the optimizationprocedure when the cost is determined to be at a satisfactory level orif the optimization procedure takes too long. If the optimization toolfinds a solution that gives a zero cost, i.e. can allocate a frequencyto all transceivers without generating any cost, the frequency optimizerstops without involvement from a user. If the optimization tool does notfind a solution that generates a zero cost it is typically adapted totry to find a better solution than the one already found.

Next, in a step, 205, when the frequency optimization procedure has beenstopped, either because a zero solution is found or because a user or apredetermined threshold level has determined to stop the procedure, theoutcome is analyzed. The analyze in step 205 typically involve ananalyze of the remaining costs, i.e. the costs that the currently lowestcost as determined by the optimization procedure generates. For example,the analyze may include looking at those remaining costs and determineif they are acceptable or not. If the costs can be accepted theprocedure proceeds to a next step 207, else if there are unacceptablecosts remaining the frequency optimization procedure in step 203 can berun again, possible with new cost weights for the different interferencecosts.

In step 207, a Base Station Identity Code (BSIC) and a TSC optimizationis performed if the optimization tool finds a solution that gives a zerocost optimizer stops without involvement from a user. If theoptimization tool does not find a solution that generates a zero cost itis typically adapted to try to find a better solution than the onealready found.

Next, in a step, 209, when the BSIC/TSC optimization procedure has beenstopped, either because a zero solution is found or because a user or apredetermined threshold level has determined to stop the procedure, theoutcome is analyzed. The analyze in step 209 typically involve ananalyze of the remaining costs, i.e. the costs that the currently lowestcost as determined by the optimization procedure generates. For examplethe analyze may include looking at those remaining costs and determineif they are acceptable or not. If the costs can be accepted theprocedure proceeds to a next step 211, else if there are unacceptablecosts remaining optimization procedure can be run again, possible withnew cost weights set in step 201 for the different interference costs.

Thereupon, in step 211, a Hopping Sequence Number (HSN) optimizationcode is performed if the optimization tool finds a solution that gives azero cost optimizer stops without involvement from a user. If theoptimization tool does not find a solution that generates a zero cost itis typically adapted to try to find a better solution than the onealready found.

Next, in a step, 213, when the HSN optimization procedure has beenstopped, either because a zero solution is found or because a user or apredetermined threshold level has determined to stop the procedure, theoutcome is analyzed. The analyze in step 211 typically involve ananalyze of the remaining costs, i.e. the cost that the currently lowestcost as determined by the optimization procedure generates. For examplethe analyze may include looking at those remaining costs and determineif they are acceptable or not. If the costs can be accepted theprocedure proceeds to a next step 215, else if there are unacceptablecosts remaining optimization procedure can be run again, possible withnew cost weights set in step 201 for the different interference costs.

Finally, in step 215, the final frequency plan including a frequencyplan and BSIC and HSN plans is determined and output from the frequencyplanning tool.

The method and system as described above is advantageously used whenfrequency planning a radio network systems with IRC. The method andsystem as described herein enables improved and optimized performance insystems with IRC (e.g. EDGE Evolution) by a method where IRCcapabilities are considered in the frequency assignment and AFP. As aresult of an improved frequency plan, an operator of the radio systemwill achieve increased bitrates, better efficiency and higher capacityin the network.

1-12. (canceled)
 13. A method of assigning frequencies in a cellular radio system where the cellular radio system is capable of Interference Rejection Combining (IRC), the method comprising: collecting data related to the cellular radio system; assigning a cost related to the number of interfering interferers; and assigning frequencies based on taking the cost related to the number of interfering interferers into account, wherein the assigned cost reduces the cost in relation to the IRC performance and the level of interference.
 14. The method according to claim 13, wherein Training Sequence Code (TSC) optimization is performed without reducing the cost in relation to the IRC performance and the level of interference.
 15. The method of claim 13, wherein the assigned cost for the number of interfering interferers imposes a cost if and only if the number of interferers is above some predetermined number.
 16. The method of claim 13, wherein, when the cellular radio system comprises IRC with two antennas, the assigned cost for one dominant interferer is reduced
 17. The method of claim 13, wherein, when the cellular radio system comprises IRC with three antennas, the assigned cost for one or two dominant interferer is reduced.
 18. The method of claim 13, wherein Training Sequence Code (TSC) optimization is performed after the frequency optimization has been performed.
 19. A device for assigning frequencies in a cellular radio system where the cellular radio system is capable of Interference Rejection Combining (IRC), said device comprising a computer configured to: collect data related to the cellular radio system; determine the Interference Rejection Combining (IRC) capabilities of the cellular radio system; assign a cost related to the number of interfering interferers; assign frequencies based on taking the cost related to the number of interfering interferers into account; and reduce the assigned cost in relation to the IRC performance and the level of interference.
 20. The device of claim 19, wherein the device is configured to perform Training Sequence Code (TSC) optimization without reducing the cost in relation to the IRC performance and the level of interference.
 21. The device of claim 19, wherein, when assigning the cost for the number of interfering interfererers, the device is configured to impose a cost for the number of interfering interferers if and only if the number of interferers is above some predetermined number.
 22. The device of claim 19, wherein the device is configured to reduce the assigned cost for one dominant interferer when the cellular radio system comprises IRC with two antennas.
 23. The device of claim 19, wherein the device is configured to reduce the assigned cost for one or two dominant interferers when the cellular radio system comprises IRC with three antennas.
 24. The device of claim 19, wherein the device is configured to perform Training Sequence Code (TSC) optimization after the frequency optimization has been performed.
 25. A method of automatically assigning frequencies in a cellular radio system that uses Interference Rejection Combining (IRC) or Single Antenna Interference Cancellation (SAIC), the method comprising: performing a frequency assignment process for cells in the cellular radio system based on optimizing cost functions that include interference cost terms that are reduced by interference reduction terms having a value dependent on IRC or SAIC interference reduction performance; and performing a Training Sequence Code (TSC) assignment process after determining frequency assignments in said frequency assignment process, wherein the TSC assignment process considers costs associated with frequency reuse between network sectors but does not consider interference cost reductions arising from the use of IRC or SAIC. 